1. Technical Field
This invention generally relates to radomes. More particularly, the invention relates to impact resistance radomes.
2. Description of the Related Art
The word "radome" dates back to World War II and is derived from the words `radar` and `dome.` Originally, radome referred to radar transparent, dome-shaped structures used to protect radar antennas on aircraft. Over time, radome has come to mean almost any structure that protects a device, such as a radar antenna, that sends or receives electromagnetic radiation, such as that generated by radar, and which is substantially transparent to the electromagnetic radiation. The structure may be flat rather than dome-shaped and may be on an aircraft, the ground or a ship.
The term "radome" as used herein in its various grammatical forms identifies any structure used to protect electromagnetic radiation equipment, e.g., radar equipment, that is aircraft, ground or ship based, unless a specific radome, e.g., or a nose radome of an aircraft, is identified.
A radome is an integral part of a radar system because the thickness of the radome and its properties determine the effectiveness of the radar and must be compatible with the specific properties of the radar set. Major design criteria of a radome include electromagnetic radiation transparency, structural integrity, environmental protection, e.g., protection from rain erosion and lightning strikes, and, especially for aircraft, an aerodynamic shape and light weight. Economics also require that the cost should be as low as possible and the service life as long as possible. Successful radome design must balance all of the conflicting requirements. For example, the ideal shape of a nose radome for an aircraft from an electromagnetic radiation standpoint is hemispherical and as large as the aircraft will allow. A better aerodynamic shape, however, is ogival. A thick radome wall would have structural benefits, yet for optimum electromagnetic transmission the wall thickness must be chosen as a factor of radar wavelength. A thin, lightweight design may improve aircraft performance, save fuel and reduce material cost but at the expense of decreased service life, increased maintenance costs and/or increased product costs. Clearly, trade-offs must be made.
Currently, a common type of radome is one having a fiberglass reinforced honeycomb core sandwich construction. The honeycomb core has an open-cell structure that encourages moisture intrusion that, as discussed below, can destroy the radome, and it has relatively poor impact resistance.
Static properties, finite element analysis (FEA), and testing traditionally have led aircraft designers to select the honeycomb core to construct the "best" radome. Although "best" is often defined as the lightest, stiffest and strongest core having the required electromagnetic properties, this approach is often inadequate, especially in impact/moisture critical environments, such as nose radomes and ship borne radomes. Radome repair data accumulated by the United States Federal Aviation Administration (FAA) indicate that about 85% of all honeycomb radomes are removed for moisture damage, and most air carriers confirm that their mean time-between-failures is substantially less than two years for some honeycomb radomes. That is to say, high maintenance costs, large inventory and questionable radar performance (due to moisture) occur.
Radomes fail when subjected to severe structural damage or degradation from electromagnetic radiation transmission. There are numerous ways for failure to occur in the hostile environment in which radomes must operate. Static electricity can cause microscopic pinholes or microcracks in a protective skin that covers the core. Lightning strikes on the outer surface of the radome can arc between the outer surface and the antenna or another electrically conductive surface to burn through the radome. Static burns are small, about the size of a pinhole or microcrack. High velocity rain or hail can cause skin laminate and core impact failure or "soft spots" in the radome which promote microcracking. Pinholes and microcracks are paths for moisture to enter the radome core. Rain or moisture causes further damage as it penetrates into the core through the pinholes, or microcracks. During the flight of an aircraft, dynamic wind pressure pumps water through the pinholes, or microcracks, deeper into the core.
Moisture in the core causes severe problems, especially if altitude or temperature changes result in multiple freeze/thaw cycles. The volume of this water expands by about 10% when it freezes causing it to exert a force against the core and skin. Repetitive freezing and thawing results in delamination, cracking and the like in the core that results in additional moisture paths and, if severe enough, radome failure. Water and ice are also detrimental to electromagnetic radiation transmission as their dielectric constant is on the order of 20 times greater than that of most materials used for sandwich construction radomes.
Another common type of radome used in aircrafts is the fluted core radome, which was adopted to combat the moisture problem associated with the honeycomb core radome. The fluted core is a series of square fiberglass tubes. Hot air is sometimes blown into the tubes to de-ice the radome and blow water away from the region of the radome where electromagnetic transmission is critical. The fluted core has an undesirably high density (approximately 200 kg/m.sup.3), which is more than twice as dense as other radome core materials. A fluted core radome also weighs approximately 30% more than its honeycomb counterpart. The construction of a fluted core radome is very labor intensive, which leads to an expensive finished product. Furthermore, repairs are expensive and time consuming. These disadvantages are not acceptable to many radome users, especially since fluted core radomes eventually retain moisture in any event.
Yet another type of radome is the foam core radome. Radomes that used polyurethane foam were popular in the 1950s, but the foam's tendency to crumble and its poor fatigue and impact properties quickly gave "foam radomes" an unfavorable reputation. Other foams that allegedly are closed-cell, e.g., polymethacrylimide foam, may actually have poor moisture absorption properties. This history of poor "foam radome" performance has hindered the development of other radomes using a better suited foam.
The use of a syntactic foam, i.e., foam containing glass microballoons, in radomes is limited because the syntactic foam radomes are heavier than honeycomb radomes.
Therefore, it would be advantageous to have a radome that overcomes one or more of the aforementioned shortcomings.